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Old 24th June 2002, 02:26 PM   #1
miguel2 is offline miguel2  Portugal
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Default How to make balanced outputs in a pre amplifier

Hi,

I have built this pre amplifier based on the EF86 tube:

http://www.tubelmmp.home.sapo.pt

It sounds well but has limited bandwith. Simulation says that it goes up to 100KHz without difficulty, but low end is quite short (80Hz at -1db) and I am thinking about the caps C15 and C19: I will try to put there about 22uF and see what hapens.
If you have some suggestions for improvement it would be nice.

But now the first question I had: how can I have balanced outputs here? Is there any way of doing this without having to dismantle everything?

Thanks.

Miguel
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Old 25th June 2002, 11:21 AM   #2
dhaen is offline dhaen  Europe
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Default Lf response

I think C10 is the problem, depending on the load impedance of your Power Amplifier as well. Try increasing it to 0.47u or 1u, ad see what happens.

A balanced output is "non trivial". You will need an extra stage.
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Old 3rd July 2002, 01:46 AM   #3
Duo is offline Duo  Canada
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I also would recommend chaning C10 to a higher value. Also, you'll neet another tube, most likely a triode to make a balanced output...
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Old 4th July 2002, 12:21 AM   #4
Pedja is offline Pedja  Serbia
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Hi Miguel,

I didnít realize, does it have limited bandwidth subjectively or you measured it (real roll-off)? If it is just subjectively, some play with coupling caps have sense. In my triode mode EF86 preamp polystyrene coupling caps open the sound.

Anyway, I think it is not necessary to use this circuit for preamp. If I realize, you have attenuation at first stage, then gain of some 50 at second stage. You can make a one stage, triode mode EF86 preamp.

Pedja
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Old 4th July 2002, 02:54 PM   #5
miguel2 is offline miguel2  Portugal
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Hi,

Thank you all for you replies.

I have no test equipment to make objective measurements on it . So my opinions are based on simulation and subjective listening.

I have played around C10 and used 0.3 uF on there and heard no diferences on the sound, playing music with a lot of bass (Celine Dion "Ballet", that has really deep bass in the middle of the song). But in the meanwhile I noted that this problem is less apparent, and this is real because I can always compare it with my integrated amp. So some "burning in" of the components must have occurred.

Pedja: I do have attenuation on the first stage, but I already tried it in triode mode (grid connected to cathode), without attenuation, and found the gain way too low. Can you send me a schematic of that 1 stage pre?

Regards

Miguel
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Old 4th July 2002, 05:34 PM   #6
dhaen is offline dhaen  Europe
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Quote:
Originally posted by miguel2
Hi,

Thank you all for you replies.

I have no test equipment to make objective measurements on it . So my opinions are based on simulation and subjective listening.

Miguel
Miguel,

Of course good sound must be the final result, but you must make some kind of measuerments, otherwise you might get lost.

You can make the basics at almost no cost:

Audio generator:
Use your PC with NCH Tone Generator: www.nch.com.au/tonegen

Buy or borrow a digital multimeter with AC volts range.
With a sinewave generated, measure the output from your soundcard with the meter to make some calibration data.

Use 1KHz as your reference, then measure the voltage levels at other frequencies.

It doesn't matter if the soundcard isn't perfect, or that the meter won't be. You can even make a speadsheet with the correction coefficients built-in.

With the above, you will be able to measure frequency response to within 1dB or better.

For a noticeable bass loss that you reported, I would guess there must be at least a 3dB loss.

NB For converting a voltage ratio to dB we use 20Log (V1/V2).

With the above set-up, there are many other measurements possible.

For convenience, or if your soundcard is very poor, you could "burn" a CDR from the NCH Generator WAV files, and use a CD player as your source - better than most soundcards.

It is probably best to generate tone at -10dB with the generator for these tests. (That's 10dB below max.)

Good luck
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Old 4th July 2002, 09:22 PM   #7
Pedja is offline Pedja  Serbia
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Miguel,

Iím not quite sure about gain you achieved with this circuit. With 1000K input resistor overall gain of your preamp is not really big (I checked your schematic in the same program you use), and hence easily achievable with one EF86 in triode mode. One solution you have at

http://www.svetlana.com/graphics/TB/no.23fig3.jpg

There are some differences in circuit I use, and regarding the gain, difference is that I donít use a cathode cap, so depending on resistors values gain can be 8-12. BTW, this way it will have wider bandwidth. But donít expect it will help with high-freq roll-off. For this problem, I really recommend trying a few caps, even high-end (film&foil) polyprop (and paper in oil also) can sound murky in this application.

Pedja
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Old 8th July 2002, 07:42 PM   #8
miguel2 is offline miguel2  Portugal
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Hi Pedja and dhaen,

Thanks a lot for your input.

I have downloaded the NHC tonegenerator and it works great. I used it with an AC multimeter and with Winscope, to see wave forms. With Winscope it is not accurate but it will do for a gross estimate. I also suspect of my cheap sound card...

So the frequency response is at -3db at about 50Hz, as you said dhaen. It is in the word file attached, curve with 22uF caps in the cathode. Then I took the caps off and saw that low frequency is good but it cuts at about 9000 Hz, which is not so good. With 0.1 uF film caps gives some nasty effects at the extremes.

So I followed Pedja's idea and put the first stage in triode mode, no feedback. In this way I had not enough gain, so I played with a second stage in cathode follower mode. The circuit is on the same file. It gave the same results as the first circuit with 22uF, as you can see in frequency graph. It also inverts the phase. I have to think a bit more, to go down in frequency. By the way, these graphs clearly show that I am having a hum problem, that I already knew because when I have the volume half way up it starts to do that terrible noise. But with a small crouded box and a 130VA transformer there I just have to think it carefully.

Regards.

Miguel
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Old 11th July 2002, 08:41 PM   #9
dhaen is offline dhaen  Europe
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Hi Miguel,

You can map the soundard's respose with your AC voltmter.

If you want to mess with cathode bypass caps, you must use a large enough value to bypass the entire audio range, otherwise you will get ringing and frequency response scew.

I still think that the output coupling cap (c10 now) is too low.
Also, what is connected to the output while you are making the measurements, and what is the total load impedance?
The AC voltmeter and winscope might be quite low (<1M ohm), in which case it will make the frequency scew seem worse.

As far as the instability is concerned, you don't tell us where the volume control is situated. Please advise.

JD
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Old 11th July 2002, 09:58 PM   #10
Pedja is offline Pedja  Serbia
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Hi Miguel,

What is the purpose of series resistor at input (R32 Ė 1000K)?

If you remove it, youíll get enough gain with one, triode mode stage, even if you lower anode resistor. That means you can remove your current second stage (it actualy doesnít have a voltage gain, but protects gain of the first stage). Voltage gain of such configuration is about 8x without, and about 20x with cathode cap. Again, why do you use that resistor?

About coupling capÖ yes, I also donít use big coupling cap, mine expirience with this tube says it is possible to use even 68nF without audible cut-off in bass. It actualy do exists, but it is not a lot, -4dB at 20Hz, Ė2dB at 30Hz, -1.3dB at 40Hz Ė all at 68K load and 47K anode resistor.

Non-linearities in freq response above 1.5KHz produced your soundboard rather than your preamp.

Pedja
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